Why no seatbelt for rear facing dinette seats?

I notice that all modern cars/SUVs/cross-overs/PU trucks have headrests on the backs of all personnel seats. Of course this make all the sense in the world.

Then .... why don't all the states in (at least) the list above also require headrests on all the forward facing, rear facing, and side facing passenger seatbacks where seatbelts are required? :h